Efficient film devices, including organic photovoltaic and organic electroluminescent devices have been the subject of much advancement recently. In particular, organic light emitting small molecules and polymers have attracted increasing interest for manufacture of large area, low cost light emitting devices. They can be used as the light-producing elements in television screens, computer displays, advertising and information board applications, and the like. OLEDs can also be used in lighting devices.
Organic light-emitting diodes (OLEDs) use an electroluminescent conductive polymer or small molecule that emits light when subjected to an electric current. They can be used to make full-spectrum color displays and require a relatively small amount of power for the light produced. No vacuum is required, and the emissive materials can be deposited onto a substrate by a technique derived from commercial inkjet printing or vapor deposition processes. The diodes can be made on either rigid or flexible substrates.
The radically different manufacturing process of OLEDs lends itself to many advantages over flat panel displays made with LCD technology. OLEDs can be printed onto any suitable substrate using inkjet printer or even screen printing technologies, which can result in a significantly lower cost than LCDs or plasma displays. In addition, printing OLEDs onto flexible substrates opens the door to new applications, such as roll-up displays and displays embedded in curtains, clothing, and the like.
OLEDs enable a greater range of colors, brightness, and viewing angle than LCDs, because OLED pixels emit light directly. OLED pixel colors appear correct and unshifted, even as the viewing angle approaches 90 degrees from normal. LCDs use a backlight and cannot show true black, while an “off” OLED element produces no light and consumes no power. Energy is wasted in LCDs because they require polarizers which filter out about half of the light emitted by the backlight. Additionally, color filters in color LCDs filter out two-thirds of the light.
As shown in the schematic in FIG. 1, an OLED 100 has an emissive layer 110, a transport layer 120, an anode 130 and a cathode 140, all on a substrate 150. The layers 110, 120 are made of organic semiconducting small molecules or polymers. When a voltage is applied across the OLED 100 such that the anode 120 is positive with respect to the cathode 140, the cathode 140 injects electrons 145 into the emissive layer 110 and the anode 130 injects holes 135 into the transport layer 120. The electrons 145 and the holes 135 move toward each other and they recombine. The recombination produces an emission of radiation 160 whose frequency is typically in the visible, may also be in the infrared and ultraviolet regions.
The electron orbitals in electroluminescent organic small molecules or polymers are analogous to the valence and conduction band edges in an inorganic semiconductor; states below the highest occupied molecular orbital (HOMO) are occupied and those above the lowest unoccupied molecular orbital (LUMO) are empty. The HOMO and LUMO are separated by an energy gap, normally in the optical energy range. Thus when an electron makes a transition from the LUMO to the HOMO, visible light can be generated.
Indium tin oxide (ITO) is commonly used as the anode material. It is transparent to visible light and has a high work function, which promotes injection of holes into the organic layer. Metals such as aluminum and calcium are often used for the cathode as they have relatively low work functions, which promote injection of electrons into the organic light emitting layer.
Unlike organic small molecules, electroluminescent polymers are long chain hydrocarbon-based conjugated molecules with molecular weights of several hundred thousand atomic units. They can be applied to substrates by spin coating or printing to form amorphous films. Typical polymers used in OLED displays include derivatives of poly(p-phenylene vinylene) and poly(fluorene). Substitution of side chains onto the polymer backbone may determine the color of emitted light or the stability and solubility of the polymer for desired performance and ease of processing.
There are several obstacles that must be overcome before the potential of OLED technology can be realized commercially. The interface between the cathode and organic layers in OLEDs presents a barrier for electron ejection, which reduces electron ejection efficiency and can lead to a significantly large device operating voltage with reduced overall device efficiency. Additionally, the barrier results in an increase in temperature at the interface which may damage the device. Temperatures can reach over 100° C., which can cause severe damage to an OLED. Another obstacle involves the stability of the OLED. Exposure to the environment (e.g., heat, H2O and O2) can be particularly damaging, leading to marked deterioration in device performance. Lastly, much of the light that is emitted by the organic molecules in an OLED remains trapped in the device and does not reach the viewer.
Generally, when the cathode and anode electrode work functions match the respective LUMO and HOMO levels in the organic material, it is easy to inject a steady supply of electron and hole pairs into the polymer to generate light. While a thin layer of transparent conductor indium tin oxide (ITO) has become the standard material for the anode, the optimal cathode material has yet to be developed. It is generally believed that, for the cathode, lowering the energy barrier between a metal contact and the LUMO of polymer promotes efficient injection of electrons. Therefore, a variety of low work function metals, especially alkali metals, have been widely used to reduce operating voltage and improve device efficiency.
Although alkali or alkali earth metals display the lowest work function (2-3 eV) of all elemental metals, thus suitable for cathode applications, their reactive nature and the associated fabrication cost have been a major challenge for the realization of low cost, high efficient OLED devices. Therefore, development of cathodes with sub-wavelength size structures such as carbon nanotubes and nanotube-alkali composite networks, and arrays of stable low work function alloy nanoclusters may improve device deficiency by lowering the driving voltage and increasing device stability and light extraction.